Project 2: Abstract Consumption of fish contaminated with mercury is a serious public health concern in the US and globally. Marine fish are the most important agents of exposure for humans to methylmercury (MeHg), the most toxic form of mercury. MeHg exposure is of particular concern for pregnant women and children and has known neurological, developmental, cardiovascular, and immunological effects. The overall goal of Project 2 is to determine how changes in multiple environmental factors will alter the production and fate of MeHg in estuarine food webs, which are important pathways of exposure to humans. We will investigate the interactive effects (additive and non-additive) of changes in temperature, salinity, and carbon content on the production and fate of MeHg in coastal marine systems in order to predict the net influence of regional variability as well as climate change on human exposure to MeHg. The project will utilize field studies and laboratory experiments to measure the effect of a range of environmentally relevant temperatures, salinities, and carbon loads on microbial activity, Hg methylation and MeHg demethylation rates, sediment MeHg content and Hg and MeHg flux from sediments to overlying water. Laboratory and mesocosm experiments will be conducted to examine the individual and combined effects of temperature, salinity and organic carbon on the bioconcentration of MeHg into phytoplankton, and trophic transfer to zooplankton and fish that are prey for higher trophic level fish consumed by humans. Field work and mesocosms experiments will be undertaken in estuaries located at different latitudes with high and low carbon concentrations in sediments in order to compare MeHg processes under different biogeochemical and temperature regimes. Data obtained from laboratory and field experiments will be used to parameterize and link current fate, transport, and bioaccumulation models with human exposure models. These combined approaches will improve our understanding of the impact of complex environmental alterations on MeHg fate and bioaccumulation in marine ecosystems, and the impact of these factors on MeHg exposure to humans.